It is known that normal human eyes can perceive light irradiation in the visible range, but with differing sensitivity at differing wavelength spans. For example, normal human eyes have the highest sensitivity at around 450 nm, 540 nm and 610 nm. Thus, it would be useful to filter off irradiation around about 500 nm and 580 nm in order to improve the perceived color contrast.
It has proven desirable to enhance the perceived color contrast by using glass materials having special filtering properties in the visible range. Some of the color and contrast-enhancing glasses as disclosed in the prior art are Nd2O3-containing glass. It is known that Nd2O3 has an absorption band between 565-595 nm.
It is also known that UV light, including UVA and UVB, are detrimental to the health of naked normal human eyes. The sunglass industry has developed and provided many versions of sunglass products to the general consumers, including polarizing products, photochromic products, and the like, many of which have UV-filtering properties.
Sunglasses that are both UV-absorbing and contrast-enhancing were proposed in the prior art before. However, some of such sunglasses were designed to have multiple layers in their structures, with at least one of them for the purpose of UV absorption, and at least one additional layer, typically bonded to the UV-absorbing layer, to perform the contrast-enhancing function. Such multi-layer structure are costly and cumbersome to manufacture, and can present stability issues, such as delamination between the layers overtime. An example of such sunglass design is provided in U.S. Pat. No. 6,604,824.
Single glass compositions having both UV-absorbing and contrast-enhancing properties were described in the prior art, such as WO 93/02109. However, the UV-absorbing and/or color filtering properties of such glasses still have room for improvement.
Therefore, there remains a genuine need of a single glass composition that is both UV-absorbing and contrast-enhancing which could be use for making sunglass lenses to protect the human vision and to improve the color perception at the same time.
The present invention satisfies this long-standing need.